1. Field of the Invention:
The present invention relates to novel stabilized polymeric compositions based on vinyl chloride, shaped articles prepared therefrom, and to a process for the stabilization of vinyl chloride polymers.
2. Description of the Prior Art:
It is known to this art that polymers based on vinyl chloride must be subjected to temperatures on the order of 160 .degree. C. to 180.degree. C., sometimes even to higher temperatures, especially during the preparation of shaped articles therefrom. To prevent or retard the various phenomena of yellowing due to these heat treatments, vinyl polymers are stabilized by the addition of various compounds thereto. Many publications feature the use of sulfur compounds for heat stabilization.
And the majority of these publications relate to organic sulfur compounds of tin, but such compounds are toxic to a greater or lesser extent.
British Patent No. 1,001,344 is directed to compositions based on polyvinyl chloride (PVC) which comprise:
(a) One or more organic compounds of lithium, sodium, potassium, magnesium, calcium, zinc, strontium, cadmium, barium, aluminum, lead or tin; PA1 (b) An organic or organometallic sulfur compound having a boiling point, under atmospheric pressure, of not less than 200.degree. C.; a compound of this type can be an alkyl or aryl sulfide, a mercaptocarboxylic acid, such as, in particular, thioglycolic acid, dithiodiglycolic acid, mercaptopropionic acid, thiodipropionic acid or dithiodipropionic acid, or esters of these acids; and PA1 (c) An organic antioxidant which inhibits the chain reactions entailing decomposition of the PVC, and which is, in particular, a substituted derivative of phenol. PA1 (i) at least one organozinc derivative; PA1 (ii) at least one organic derivative of a metal from Group IIa of the Periodic Table (such as published, for example, in the 51st edition of Handbook Of Chemistry And Physics, published by The Chemical Rubber Company); and PA1 (iii) at least one thiomalic acid diester.
It is very difficult to select, from among the innumerable sulfur compounds which fall within the broad definition in said '344 patent, those which do in fact exert a heat-stabilizing influence.